Safety paper and method of making same



certificates, or other instruments,

Patented'Apr. 21, 1936 UNITED fsTATEs Mame:

riii'nn AND nm'r'non OF MAKING SAME George Ll Monte notation oi New Jersey L. Simona, Noedham, Mala, casino: to & Son, Nutley,

N. 1., a cor- No Drawing. Application May 9, .1933,

Serial No.

28 Claims. ((31. 91-6195) I This invention relates to safety paper, includ ing fraud preventing writing material made of paper or other substance or composition adapted ior printing or lithographing or for the inscrintion oi writing thereon and for use, ior example, in the manuiacture oi? checks, draits, bonds, stock which it is de sired to have protected against fraudulent alters tion. This invention also relstes to methods oi melting such. paper.

This invention relates particularly to ssiety paper manufactured using o ierrocycnide es s safety chemical. i errccysnides when incorporated in safety no oer boys the property oi iormius with iron cons pounds in ink certein deeply colored compounds known as Prussian blue, the onset chemical nature of which. is not certain end may very to some cutout depending upon the proportions end clierecter oi the reacting substances. Prussisiu blue is highly resistant to the ml; erodicotors in. general use at the rare-sent time. The most commonly used inlr eredicetors hove s. chlorine bleach e. basis. Another type of erodicotor hes pots-s slum permsuuenste es obesis. The letter erseii caster is normally ioliowecl in ectuel use by the amelioration oi some reducing scent such us so dium bisulphite to clean up the monsenese di oxide deposited during the epplicetion oi the permongenote. The Frussion blue formed. by writing with ordinary iron-containing ink on. mistyoner prepared with ierrocyonides is re sisteut to the action oi both oi the ercdicotors referred to. Moreover, upon the opplicstiou oi? 1 wins to ssiety oner thus prepared, so, for on simple, iii treating ink preliminarily for the s sequent euplicsitiou of s. chlorine bleach type oi erouicotor, the ierrocysuide is causes to decom= pose with the iotlon oi Prussisn blue as on indelible disfiguring mark or stein. Such ireud. preventing characteristics of ierrocyonides ere sftlorded when ierrocyonidec are applied to satiety genes in efi'ective amount es an ink s scent. Seiety oners conte soluble ierrocyenioles, use subject, however, to the. very serious defect that they sraduolly become discolored in the presence of sir end/or light to produce cremeturely objectionable stains and discolorotions on soiety paper. The discoloration of the ierrocy snide is believed to be due to o gradual decomposition of the ierrocyanide with resul'libers tion of iron compounds'that in turn react with. the remainder of the ierrocyanide forming Pros sian blues or other colored compounds." While the decomposition of the ferrocyanide and resultantdiscoloration can be inhibited to a certain extent "by the inclusion oi alkali or alkaline substances in safety paper, the presence of the alkali in the paper results in almost complete loss of the sizing and waterproof characteristics of the paper. The rosin which is normally used in sizins paper is acid in character and even in the presence of weak alkali its sizing ability is promptly destroyed with the result that the sheet lee-- cornes' absorbent, causing blotting end ieether- 10 inc of ink applied thereto. Moreover, alkali. is not e, wholly satisfactory stabilizer, as a. sheet of paper treeted with s soluble ierrocyeniole and an alkali will eventually turn blue on standing, or

in light, or more rapidly whensubjected to both 5 conditions. While attempts have been made toovercoxne the desizing efiect of alkaline stabilizers by the use of secondary sizing materials such as stercii, casein, end glue, such attempts have not resuiteol in coercially satisfactory products. 20

it is s. feature oi this invention that it is not necessary to use a secondary sizing material to counteract any desisinu efifect such as would be ceusecl by slimline stabilizers.

it is e purpose oi this invention to afford. a. 5 ierrocyonide saiety'pener which is satisfactory iroui the point of view of its resistance to discoloration upon aging in sir end/or upon exposure to strong light and which is also satisfactory iroin the point of view oi its writing propero ties.

According to this invention, it ierrocyanide seiety ssuer is tfiorded which does not blot or on feathering when ink is applied thereto and which is more perfectly discoloration than any which has been tureci heretofore. The ierrocyariides which are particularly susceptible to decomposition ere the water soluble ierrocyenides, namely, the ferrocyanides of 9.1- so keli metals (including ammonium) and. of a1- ltsline earths. Such ierrocyanides will be reierrecl to herein as soluble ferrocyenides. The use of soluble ierrocyanioles is preferable in the prectice of this invention because they are co- 4.5 pubic of ready application to safety paper and because they are much more effective in fixing ink applied to safety paper than are the insoluble :ierrocynnides. The ferrocyariides of sodium or potassium effective and are relatively inexpensive.

To eflord a. clear understanding of this invention, it will now be described in connection with I certain specific examples illustrative thereof.

in one embodiment oi this invention, safety 55 protected against 35 are regarded. as being particularly so within the scope ofthis invention.

paper is manufactured which has incorporated therein not only a soluble ferrocyanide but also a. substantial amount of alkali metal phosphate. The phosphates of alkali metals (including ammonium phosphate) have been found to possess is evidenced by the fact that tri-sodium phosphate exerts a decidedly greater retarding effect on the decomposition of soluble ferrocyanide than do other substances of equal alkalinity.

The mono-sodium phosphate has a slightly acid reaction in aqueous solution. The acidity of the mono-sodium phosphate causes ferrocyanide cyanide in safety paper.

Di-sodium phosphate when made into an aquephosphate' or phosphoric acid. Thus, the pH value of the solution can be reduced to about 7.2, which indicates a practically neutral solution.

condition in safetypaperis to be regarded as Safety paper prior .to treatment with ferrocyanide solution andwith alkali metal phosphate, is normally slightly acid due largely to the acidity of the sizing material. Thus, for example, the

' of the solution require.

' phosphate.

paper by itself may have a pH value of about 4.7 to about 6.5. In such case when the paper is about 6.2 and suits.

As a specific illustration of this invention, a bath is made up containing -10% sodium ferroe, 12% di-sodium phosphate and 1% monosodium phosphate, the balance of the solution being water. The proportions given are based upon the weight of the fully hydrated commercial salts that are added to the impregnating bath. The acidity of the solution thus made up is in the neighborhood of 7.5 on the pH scale. The solution can be applied in the manufacture of safety paper in any suitable manner. For example, the paper can be passed through a bath and then subjected to squeeze rolls to squeeze out excess moisture. The paper thus treated can then be dried and finished in any suitable manner, as is well known in the art.

Due to the fact that the paper itself is slightly acid as stated above, the finished paper will be very nearly neutral. As the bath becomes depleted, it, of course, can be renewed as the needs In this connection, it has been found that the bath takes up a slight degree of acidity from the paper and to offset this it is preferable to replenish the bath with a. solution which has a pH value that is about .2 to about .3 higher than pH value desired in the bath that is applied 7 directly to the paper.

pregnating bath "or can be applied separately as v by separate baths. Whether the ferrocyanide and/or stabilizer therefor are applied through the material or at the surface, the same will be regarded as incorporated with the safety paper.

It is to be noted that the above solution contains a substantial quantity of di-sodiurnphosphate and substantial quantity of mono-sodium The mixture of the two phosphates or of the phosphates with phosphoric acid in the finished paper has been found to act in the capacity of an acid-alkali buffer giving increased resistance to the action of acids or alkalies. Thus,

the finished paper is not easily discolored by air containing gases such as S0: or by waste gases of acid or alkaline nature. The inclusion of such buffer with the safety paperis to be regarded as an advantage and; feature of this invention. Moreover, the presence of the buffer in the impregnating bath contributes to the maintenance of uniform alkali-acid conditions which otherwise might be aifected to undesirable extent by such causes as bringing different rolls of paper having different degrees of acidity into the bath. Another buffer combination is a solution of mixed sodium sulphite and sodium bi-sulphite. The

I above described.

latter substances also exhibit some stabilizing effect on ferrocyanides, as well.

To give another specific example of an applithe illustration just given, the phosphoric acid is used to reduce the normal alkalinity of disodium phosphate solution to approximately the neutral point. This impregnating bath may be applied in the manufacture of safety paper as Alkali metal ferrocyanides are practically neutral in themselves. Thus the chemical substances incorporated in the safety paper including both the ferrocyanide and the protecting agent therefor, in the abovespeciflcally illustrating embodiments of this invention, are, taken altogether,

about neutral.

Preferred results can be produced in the practice of this invention by controlling the pH value of the incorporated substance between 6.8 on the acid side and about 7.5 on the alkaline side. However, the advantages of this invention re sulting from keeping the impregnated material at about the neutral point can be largely afforded by controlling the pH value of the incorporated substances between about 6.7 on the acid side and about 8.5 on the alkaline side.

In preparing safety paper with ferrocyanide, it is usually preferable to employ enough of the ferrocyanide to exhibit a pronounced ink fixing reaction with the iron in inks which may be applied thereto and to give a pronounced discoloration if acids are applied to the paper. The employment of asolution of ferrocyanide of approximately 10% concentration will afford a bath which will incorporate in the safety paper a suitable amount of ferrocyanide to produce the desired protection. The finished paper usuallycontains about /th the proportion of ferrocyanide and stabilizing agent which is contained in the impregnating bath. Usually about /2% to about 2% of ferrocyanide should be incorporated in the manufactured safety paper to impart very pronounced fraud preventing characteristics to the paper. The amount of ferrocyanide that is incorporated in the paper may be increased to about 5% which approaches the maximum amount that thepaper will retain when impregnated with a saturated solution of ferrocyanide. The more ferrocyanide that is retained in the paper the more vigorous the ink fixing and stain producing reactions become. invention that the ferrocyanide is stabilized to a commercially satisfactory extent 'even though a maximum amount of ferrocyanide is incorporated in the safety paper. For purposes of economy, lesser quantities of ferrocyanide such as the quantities above suggested may be used and have been found to give satisfactory ink fixing and stain producing properties to safety paper.

The amount of alkali metal phosphates which should be used to stabilize the ferrocyanide is subject to considerable variation. More than about 5% by weight in the bath of the alkali metal phosphate should preferably be added to a solution of ferrocyanide to give desired stability to the ferrocyanide in the finished paper as the amount of phosphate stabilizer should preferably be not less than about half the amount of ferrocyanide to afford satisfactorily permanent stabilization. Preferably the amount of alkali metal phosphate used should be about 50% to be used except that coloration caused It is a feature of this quantities e 150% on the weight of the ferrocyanide. Increased amounts of the phosphates give greater stability. There is probably no precise upper limit as to the amount of phosphate which may by the use of excessive amounts of phosphate, the cost of the safety paper may become unnecessarily increased. Usually about 12% to about 14% of alkali metal phosphate in a 10% solution of sodium ferrocyanide gives very excellent stabilization of the ferrocyanide and safety papers made from such an impregnating bath not only exhibit in highdegree the safety features of ink fixing and stain production but also are highly resistant to subsequent discoloration due to ferrocyanide.

A' safety paper containing ferrocyanide, stabilized by alkali metal phosphates can be further improved by incorporating in the safety paper an alkali metal fluoride, e. g. sodium fluoride. Alkali metal fluoride exhibits the propertyof preventing iron compounds which result from the decomposition of ferrocyanide in safety paper,

from reacting with undecomposed ferrocyanide and forming colored compounds. Thus the alkali metal fluorides have to a considerable degree the capacity of stabilizing ferrocyanides. While the employment of alkali metal fluoride with sodium phosphate is desirable in the practice of this invention, the employment of alkali metal fluoridealone as a' stabilizer for ferrocyanide in safety paper is to be regarded as one of the features of this invention.

When sodium fluoride, for example, (which in itself is approximately neutral) is used in .connection with the employment of. alkali metal phosphates to stabilize ferrocyanide in safety paper, the employment of a bath having slightly greater acidity than would be possible in the absence of sodium fluoride, is permissible. Thus a fairly high degree of protection against disby decomposition of ferrocyanide can be afforded by adding about 1% of sodium fluoride to the impregnating baths given in the foregoing illustrations of the practice of this invention even though the pH value of the bath is as low as about 6. In the finished paper the pH value, when an alkali metal fluoride is used, preferably ranges from about 6 to 1.1.

The amount of alkali metal fluoride which need be used in the practice of this invention is relatively small. For example, about 1% of sodium fluoride in a bath which is applied to the paper is sufficient to give the desired results... Lesser decreasing stabilizing effect. Greater quantities do not result in substantially greater stabilizing eifect. Moreover, it is ordinarily preferable to employ less than about 0.5% of alkali metal fluoride as the employment of substantially greater quantities than 1% in the bath results in a slight and gradual desizlng effect-in the finished paper.

While it is regarded as preferable to employ alkali metal phosphates as stabilizing agents in preventing the decomposition of ferrocyanide in safety paper, it has been found according to this invention that considerable stabilizing effect is I action, their alkaline or salt be used or unless the color is suitably camouflaged or is regarded as desirable. It is to be understood that ammonium salts are not included among the alkali metal and alkaline earth salts other than phosphates which may be employed according to this invention.

If any of the salts above mentioned which may be used according to this invention either have a definitely acid or definitely .alkaline reacid characteristics should be neutralized so as to bring: the pH value of the solution thereof within about 6.7 on the acid side and about 8 on the alkaline side, except that when the alkali metal fluoride is used with any of the salts, the resulting solution can be used in a somewhat more acid state, namely, having the pH value above about 6. Moreover, such salts should not react with the ferrocyanides as by causing the oxidation or reduction of the ferrocyanides.

When other alkali metal or alkaline earth salts are used than alkali metal phosphates (which latter are regarded as preferable) relatively large quantities of the salts should be used to produce substantial stabilization of the ferrocyanide. About 50% to 150% of such salts in, proportion to the weight of the ferrocyanide should preferably be incorporated in the paper. The pH value of the finished paper should preferably be about 6.2 to about 7.3.

This invention has heretofore been described in connection with the. stabilization of soluble ferrocyanides employed i connection with safety paper. If an insoluble ferrocyanide is employed, such as manganese ferrocyanide as by the precipitation of manganese ferrocyanide from a solution of soluble ferrocyanide incorporated with the paper, such paper can be greatly improved by employing the stabilizing chemicals hereinabove described. In the method of preparing safety paper using an insoluble ferrocyanide formed by impregnation with successive baths, it is difficult to bring about complete interaction between the soluble ferrocyanide and the soluble manganese salt, whereby free soluble ferrocyanide is frequently left in the paper. The incorporation of 'the stabilizing chemical hereinabove described serves to prevent the discoloration of the safety paper due to the decomposition of otherwise spontaneously decomposable ferrocyanide in the safety paper. t

While this invention has been described in connection with certain specific examples thereof, it is to be understood that this has been done for the purpose of illustration and that the scope of this invention is not to be limited thereby. It is also to be understood that this invention is capable of other applications than those herein given by way of illustration.

I claim:

1. As an article of manufacture, a safety paper having incorporated therewith a ferrocyanide and an alkali metal phosphate.

2. As an article of manufacture, va safety paper having incorporated therewith a soluble ferrocyanide and an alkali metal phosphate.

3. As an article of manufacture, a safety paper having incorporated therewith a soluble ferrocyanide and a substance selected from the group consisting .of sodium and'potassium phosphate.

4. As an article of manufacture, a safety paper having incorporated therewith a ferrocyanide, an alkali metal fluoride, and an alkali metal phosphate.

5. As an article of manufacture, a safety paper substances applied to having incorporated therewith a soluble ferro cyanide, sodium fluoride, and a substance selected from the group consisting of sodium and potassium phosphate.

' 6. As an article of manufacture, a' safety paper having incorporated therewith a ferrocyanide and an alkali metal fluoride. I

7. As an article of manufacture, a safety paper having incorporated therewith a soluble ferrocyanide and a substance selected'from the group consisting of sodium and potassium fluoride.

8. As an article of manufacture, a safety paper comprising a soluble ferrocyanide and an alkali metal phosphate, the pH value of the finished product being between about 6.2 and about 7.3.

9. As an article of manufacture, a safety paper comprising a soluble ferrocyanide and asub-. stance selected from the group consisting of sodium and potassium phosphate, the pH value of the finished product being between about 6.5 and about 7.0.

10. As an article of manufacture, a safety paper comprising a soluble ferrocyanide, sodium fluoride, and a. substance selected from the group consisting of sodium and potassium phosphate, the pH value of the substances embodied in said sheet being between about 6.0 and about 7.1.

11. As an article of manufacture, a safety paper containing a soluble ferrocyanide, and disodium phosphate. I

12. As an article of manufacture a safety D8- per comprising a soluble ferqicyanide, til-sodium phosphate, and mono-sodium phosphate.

13. As an article of manufacture, a safety paper comprising a. soluble ferrocyanide and an acid-alkali buffer, the pH value of the finished product being between about 6.2 and about 7.3.

14. As an article of manufacture, a safety pa-. per comprising a soluble ferrocyanide and a substance selected from the group consisting of soluble salts of alkaline earthmetals and of alkali metals other. than an ammonium, the pH value of the finished product being between about 6.2 and about 7.3.

15. As an article of the pH value of the substances incorporated in said sheet being between 6.0 and 7.1.

16. As an article of manufacture, a safety paper comprising incorporating therewith about to about 5% of a soluble ferrocyanide and an alkali metal phosphate, the amount of alkali metal phosphate being about 50% to about 150% of the weight of the ferrocyanide.

17. As an article of manufacture, a safety paper comprising a soluble ferrocyanide and a sub- 6.2 and about 7.3.

18. A method of making safety paper which comprises incorporating with the paper waterdissolved ferrocyanide and a water-dissolved salt of a metal selected from the group consisting of alkaline earth metalsand alkali metals other than ammonium, the pH value of the dissolved the paper being between about 6.7 and about 8.5 and removing excess water. 1

19. A method of making safety paper whichv comprises incorporating with the paper waterdissolved ferrocyanideand a-water-dissolved salt of a metal selected from the group consisting of. alkaline earth metals and alkali metals other than ammonium, the pH value of the dissolved substances applied to the paper being between about 6.7 and about 8.5 and removing excess water, the pH value of the paper prior to the application of said dissolved substances thereto being between about 4.7 and about 6.5.

20. A method of making safety paper which comprises incorporating with paper water-dissolved ferrocyanide and water-dissolved alkali metal phosphate and removing excess water.

21. A method of making safety paper which comprises incorporating with paper water-dis- I solved ferrocyanide and water-dissolved alkali metal phosphate, the pH value of the dissolved substances as applied to the paper being between about 6.8 and about 8.

22. A method of making safety paper which comprises incorporating with paper water-dissolved ferrocyanide and water-dissolved alkali metal phosphate, the pH value of the dissolved substances as applied to the paper being between about 6.8 and about 8, the pH value of the 25. A method of making safety paper which comprises incorporating with paper water-dissolved ferrocyanide and'a water-dissolved mix-. ture of di-sodium phosphate and mono-sodium phosphate and removing excess water. I

26. As an article of manufacture, a safety paper comprising a soluble ferrocyanide, a size decomposable in the presence of a substantial amount of alkali, and a stabilizer for ferrocyanide, the pH value of the finished product being between about 6.2 and about 7.3. 27. As an article of manufacture, a safety paper comprising a soluble ferrocyanide and an acid alkali buffer including sodium sulphiteand sodium bisulphite the pH value of the finished product being between about 6.2 and about 7.3. 28. A method of making safety paper which comprises incorporating with the paper water dissolved ferrocyanide and as astabilizer for the ferrocyanide a water-dissolved salt of a metal selected from. the group consisting of alkaline earth metals and alkali metals other than ammonium, the pH value of the bath of dissolved substances applied to the paper being between about 6.7 and about 8.5 and removing excess wa ter, the pH value of the paper prior to the application of said dissolved substances thereto being between about 4.7 and about 6.5,- and replenish ing the impregnating bath with fresh quantities of dissolved ferrocyanide and dissolved stabilizer, the pH value of replenishing materials being about .2 to about .3 greater than the bath.

FRANCIS L. SINIONS. 

